In cellular networks for wireless communication, interference often occurs in a cell caused by signals transmitted to or from User Equipments, UEs in nearby located cells, which is a well-known problem. In such a network, the available radio bandwidth is limited and in order to provide capacity for communications in the network having multiple cells, resources pertaining to radio bandwidth must be reused in cells at a sufficient mutual distance so as not to disturb communication for one another. In this context, cells that are located near a serving cell are generally referred to as “neighbouring cells” and this term will be used here in the sense that transmissions in neighbouring cells may potentially disturb transmissions in the serving cell, and vice versa, thus causing interference. It should be noted that a neighbouring cell is not necessarily located right next to the serving cell but may be located one or more cells away, still causing interference.
This disclosure is relevant for cellular networks using any of the following radio access technologies: Orthogonal Frequency Division Multiplexing (OFDM), Single Carrier-Frequency Division Multiple Access (SC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Time Division Multiplex (TDM), and Frequency Division Multiplex (FDM). Further, resources pertaining to radio bandwidth will be referred to as “bandwidth resources” for short. Typically, bandwidth resources can be defined by any combination of frequencies and time intervals. In systems of Long Term Evolution, LTE, the bandwidth resources are known as Physical Resource Blocks, PRBs, defined by frequency and time interval, and in the following text bandwidth resources can be understood as PRBs in an LTE context.
A general problem in cellular networks is that the performance in communications may be degraded and the network capacity may also be reduced, due to interference when the same bandwidth resources are reused in multiple nearby cells. This problem is common for so-called “cell edge UEs” located near the cell border but relatively far from the serving base station when the latter is situated at the cell centre, thus requiring relatively high transmit power and also being close to neighbouring cells in the vicinity, as opposed to so-called “cell centre UEs” located closer to the serving base station which therefore need less transmit power for proper communication.
A serving base station may also be located at one end or corner of the cell, e.g. when forming a sector cell or the like. In that case, a cell-edge UE located near the opposite end or corner of the cell will be “far” from the base station but “close” to an adjacent neighbouring cell, relatively speaking. On the other hand, a cell-edge UE might still be physically close to the base station but in the intersection between two cells. The terms “cell edge UEs” and “cell centre UEs” will be used here in a relative sense and without limitation to any particular distance to the serving base station and cell border, respectively, wherein cell edge UEs are more inclined to cause inter-cell interference than cell centre UEs.
FIG. 1 illustrates an example with two neighbouring cells A and B having radio coverage provided by a first base station 100A and a second base station 100B, respectively, both being situated at respective cell centres in this case. In cell A, a cell centre UE 102 and a cell edge UE 104 transmit uplink data signals x and y, respectively, where signal y is stronger than signal x in this case mainly due to the difference in distance to their serving base station 100A. UE 104 is also located close to cell B. The figure also illustrates that another UE 106 in cell B transmits an uplink data signal z using bandwidth resources that coincide with at least those used by the UE 104, and signal z may therefore be interfered by the uplink transmission of signal y from UE 104 when received by the second base station 100B, indicated by an interfering signal y′. Signal x will likely not cause any interference in cell B in this case since it is transmitted with substantially less power, and also because UE 102 is located farther away from base station 100B than UE 104.
In order to address such interference related problems, several Radio Resource Management, RRM, mechanisms have been devised. Among others, so-called “Inter-Cell Interference Coordination, ICIC” can be employed where neighbouring base stations exchange information, e.g. over an X2 interface defined in LTE, e.g. in order to coordinate their use of bandwidth recourses. Another RRM mechanism is using power control such that the power used for signal transmissions, either uplink or downlink, is controlled to be no higher than needed for proper signal detection, thus not causing interference to no avail. Basically, ICIC refers to coordinated schemes dependent of information exchange between base stations, while power control can be used more autonomously. Some examples of ICIC schemes are briefly outlined below with reference to FIG. 2 illustrating exchange of information between the base stations 100A and 100B of FIG. 1.
A so-called “High Interference Indicator, HII”, referring to uplink resource allocations for UEs in a serving cell, may be sent to the base stations of one or more neighbouring cells. The HII parameter indicates that a certain set of uplink bandwidth resources, e.g. PRBs, will be allocated to UEs in the first cell which potentially cause high inter-cell interference, such as cell edge UEs requiring high transmission power. A neighbouring base station receiving the HII is thus able to avoid using the same bandwidth resources in its own cell, if possible. In FIG. 2, base station 100A sends the HII to base station 100B in an action 2:1a, indicating that transmissions on a particular resource, such as signal y, are likely to cause interference in neighbouring cells. Another action 2:1b indicates with dashed arrows that base station 100A sends this HII to other neighbouring base stations as well, not shown.
A so-called “Interference Overload Indicator, IOI”, referring to uplink interference experienced in a cell, may further be sent from a base station of that cell to the base stations of one or more neighbouring cells. The IOI parameter basically indicates that a certain level of interference is currently experienced on a set of bandwidth resources in the cell. It is common to indicate this level as high, medium or low. In response thereto, a base station receiving the IOI can thus reduce the inter-cell interference by allocating a different set of resources to its own UEs. In FIG. 2, base station 100B sends the IOI to base station 100A, in an action 2:2a, indicating that a certain level of interference is experienced in the cell of base station 100B on certain bandwidth resources. This interference may be caused by signal y′ and/or by other signals in other cells as well, and base station 100B does not really know which cell(s) the interfering signals come from. Another action 2:2b indicates with dashed arrows that base station 100B sends this IOI to other neighbouring base stations as well, not shown. The HII can be seen as a proactive RRM mechanism while the IOI is a reactive one.
The above RRM mechanisms entail various restrictions in the usage of radio resources to limit the effects of interference between cells, also resulting in reduced capacity in the cells involved. It is a problem that the above RRM mechanisms and others are sometimes employed without much effect on the interference between cells, while still significantly reducing capacity in the cells.
As mentioned above, it is possible for base stations to avoid using bandwidth resources being subject to interference, e.g. as indicated by the above HII or IOI, and therefore often re-allocate any UEs using interfered resources. This behaviour may be helpful but sometimes results in frequent re-allocation of resources back and forth, more or less in a “ping-pong” manner, without really solving the interference problem, instead causing instability, deterioration in service quality, and excessive signalling and processing.